This invention relates to a numerically controlled feeding apparatus, and more particularly to a numerically controlled feeding apparatus having a correcting system for a mechanical motional error of the feeding apparatus.
A numerically controlled feeding apparatus has been used to feed various members such as a machining member, a cutting tool, a plotter or the like in one or higher dimensional direction. This numerically controlled feeding apparatus includes, for example, a feeding table for supporting the machining member or the like thereon, a guide surface for guiding the feeding table therealong, a ball thread for moving the feeding table on the guide surface; and other members. This type of feeding apparatus has been generally accompanied by a positioning or feeding error, which frequently occurs in a combined fashion of a pitch error of the ball thread, an unflatness error of the guide surface, an Abbe's error and so on.
In order to correct the positioning or feeding error as described above, there has been proposed a correcting device in which a difference in a feeding direction between an indicated position of a numerical position indicating device and a real position of the machining member or the like is stored as an error correcting value in a memory and then the error correcting value is added to an instruction value representing the indicated position to thereby control the movement of the machining member. This correcting device is described in Japanese Examined Published Patent Application No. 59-11125 and U.S. Pat. No. 3,555,254.
FIG. 1 is an explanatory diagram for explaining the error correcting value as described above, particularly in a case where an uniaxial or one-dimensional linear feeding apparatus is used. As shown in FIG. 1, a feeding table 102 for mounting a machining member or the like thereon is provided in such a manner as to be movable on a guide surface 101, and fed along the guide surface 101 by a ball thread having a central axis 104 in accordance with an instruction value from the numerical position indicating device. In this case, a positioning or feeding control is carried out at each of positions P1, P2 and P3 on the central axial 104 of the ball thread whose direction corresponds to a feeding direction, in other words, the positioning is carried out at each of the stop positions Q1, Q2 and Q3 of the machining member or the like (not shown) on a position estimation surface (ordinarily, a surface of an object 106 to be machined).
As shown in FIG. 1, the stop positions Q1, Q2 and Q3 at which the machining member is really stopped on the estimation surface is not consistent with indicated positions R1, R2 and R3 at which the machine member is instructed, in advance, to be stopped because the pitch error, the Abbe's error, etc. occur. In this case, a difference in the feeding direction between each of the stop positions Q1, Q2 and Q3 and each of the corresponding indicated positions R1, R2 and R3 corresponds to each of positioning or feeding errors e1, e2 and e3 in the feeding direction.
The positioning errors e1, e2 and e3 occur in combined fashion with pitch errors p1, p2 and p3 of the ball thread, and the Abbe's errors a1, a2 and a3. The pitch error is caused by variation in pitch interval of the ball thread, and the Abbe's error is caused by deviation of a plane (A) vertical to the guide surface 101 from a plane B vertical to the feeding direction due to unflatness of the guide surface), in other words, by a meandering movement of the feeding table 102 on the vertical plane B. The positioning or feeding errors e1, e2 and e3 are measured beforehand and stored as error correction values.
In an uniaxial (one-shaft) linear feeding apparatus, a correcting operation is carried out by adding each error correction value to an instruction value representing each of the stop positions Q1, Q2 and Q3. Further, in the biaxial (two-shafts) linear feeding apparatus, the correcting operation is carried out every feeding direction for each shaft.
On the other hand, in the uniaxial linear feeding apparatus, the feeding error also occurs in a direction (Y-axis) intersected (for example, perpendicular) to the feeding direction (X-axis) due to the meandering (unevenness or unflatness) of the guide surface 101. That is, the meandering (unflatness) of the guide surface 101 in a feeding direction (axis) causes not only a feeding error in the direction, but also another feeding error in a direction intersected (for example, perpendicular) to the direction. In other words, a feeding operation on a feeding axis causes at least two feeding errors on the feeding axis and another feeding axis different therefrom (or intersected thereto).
In the conventional linear feeding apparatus having two or more feeding axes (shafts) which are intersected to each other at an angle, a feeding error on each feeding axis is independently corrected using a corrected feeding amount with respect to only the feeding axis although the feeding error on a feeding axis is caused by not only a feeding operation on the feeding axis, but also a feeding operation on another feeding axis, that is, the feeding errors on all the feeding axes are not autoerrors which are independent of one another, but correlated errors which are related to one another.
Accordingly, in the conventional numerically controlled feeding apparatus having at least two linear feeding devices whose feeding directions are intersected to each other, a feeding error of one linear feeding device which occurs in one feeding direction includes at least one component of a feeding error of the other feeding device (for example, an error component occurring on the Y-axis which is caused by the feeding operation on the X-axis), so that a feeding error in a feeding direction has a complicated mechanical error. However, this complicated mechanical error can not be completely corrected in the conventional feeding apparatus because each linear feeding device carries out a correcting operation for a feeding error (autoerror) thereof independently of the other feeding errors which are caused by the other linear feeding devices.